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Analytical and Bioanalytical Chemistry (v.370, #6)
Comparison of detection limits in environmental analysis – is it possible? An approach on quality assurance in the lower working range by verification by S. Geiß; J. W. Einax (pp. 673-678).
Detection limit, reporting limit and limit of quantitation are analytical parameters which describe the power of analytical methods. These parameters are used for internal quality assurance and externally for competing, especially in the case of trace analysis in environmental compartments. The wide variety of possibilities for computing or obtaining these measures in literature and in legislative rules makes any comparison difficult. Additionally, a host of terms have been used within the analytical community to describe detection and quantitation capabilities. Without trying to create an order for the variety of terms, this paper is aimed at providing a practical proposal for answering the main questions for the analysts concerning quality measures above. These main questions and related parameters were explained and graphically demonstrated. Estimation and verification of these parameters are the two steps to get real measures. A rule for a practical verification is given in a table, where the analyst can read out what to measure, what to estimate and which criteria have to be fulfilled. In this manner verified parameters detection limit, reporting limit and limit of quantitation now are comparable and the analyst himself is responsible to the unambiguity and reliability of these measures.
Quantifying the measurement uncertainty of results from environmental analytical methods by Johann Moser; W. Wegscheider; Constanze Sperka-Gottlieb (pp. 679-689).
The Eurachem–CITAC Guide Quantifying Uncertainty in Analytical Measurement was put into practice in a public laboratory devoted to environmental analytical measurements. In doing so due regard was given to the provisions of ISO 17025 and an attempt was made to base the entire estimation of measurement uncertainty on available data from the literature or from previously performed validation studies.Most environmental analytical procedures laid down in national or international standards are the result of cooperative efforts and put into effect as part of a compromise between all parties involved, public and private, that also encompasses environmental standards and statutory limits. Central to many procedures is the focus on the measurement of environmental effects rather than on individual chemical species. In this situation it is particularly important to understand the measurement process well enough to produce a realistic uncertainty statement. Environmental analytical methods will be examined as far as necessary, but reference will also be made to analytical methods in general and to physical measurement methods where appropriate.This paper describes ways and means of quantifying uncertainty for frequently practised methods of environmental analysis. It will be shown that operationally defined measurands are no obstacle to the estimation process as described in the Eurachem/CITAC Guide if it is accepted that the dominating component of uncertainty comes from the actual practice of the method as a reproducibility standard deviation.
Uncertainty of measurement. Twenty years afterwards by Yuri I. Alexandrov (pp. 690-693).
The application of the concept “uncertainty” causes considerable difficulties. In this paper an analysis of the intrinsic contradictions of the concept itself and its relationship with the statements of metrology is given with the aim of establishing possible reasons for these difficulties. As a result of this analysis several examples are presented to demonstrate the conflicts of the concept and its several statements in the general fundamental notions of metrology.
Resolution of two-way data: theoretical background and practical problem-solving Part 1: Theoretical background and methodology by Yizeng Liang; O. M. Kvalheim (pp. 694-704).
This paper reviews recent progress in the resolution of two-way data obtained from hyphenated instruments. Special emphasis is placed on the solution of practical problems. Methods for estimating the number of chemical components both statistically and visually (the first step in solving the resolution problem) and methods for resolving the pure profiles (the second step in solving the resolution problem) are discussed in detail. To deal with real-world problems, pitfalls in the chemometric analysis of the two-way data from the instrumental measurements are also pointed out. Applications of methods for solving some difficult practical problems in environmental chemistry, pharmaceutical chemistry, and physical chemistry will be discussed in the second part of this paper.
Some case studies of skewed (and other ab-normal) data distributions arising in low-level environmental research by L. A. Currie (pp. 705-718).
Three general classes of skewed data distributions have been encountered in research on background radiation, chemical and radiochemical blanks, and low levels of 85Kr and 14C in the atmosphere and the cryosphere. The first class of skewed data can be considered to be theoretically, or fundamentally skewed. It is typified by the exponential distribution of inter-arrival times for nuclear counting events for a Poisson process. As part of a study of the nature of low-level (anti-coincidence) Geiger– Müller counter background radiation, tests were performed on the Poisson distribution of counts, the uniform distribution of arrival times, and the exponential distribution of inter-arrival times. The real laboratory system, of course, failed the (inter-arrival time) test – for very interesting reasons, linked to the physics of the measurement process. The second, computationally skewed, class relates to skewness induced by non-linear transformations. It is illustrated by non-linear concentration estimates from inverse calibration, and bivariate blank corrections for low-level 14C–12C aerosol data that led to highly asymmetric uncertainty intervals for the biomass carbon contribution to urban “soot”. The third, environmentally skewed, data class relates to a universal problem for the detection of excursions above blank or baseline levels: namely, the widespread occurrence of ab-normal distributions of environmental and laboratory blanks. This is illustrated by the search for fundamental factors that lurk behind skewed frequency distributions of sulfur laboratory blanks and 85Kr environmental baselines, and the application of robust statistical procedures for reliable detection decisions in the face of skewed isotopic carbon procedural blanks with few degrees of freedom.
Multivariate statistical interpretation of coastal sediment monitoring data by V. Simeonov; I. Stanimirova; S. Tsakovski (pp. 719-722).
Multivariate statistical analysis of sediment data (input matrix 122 × 15) collected from 122 sampling sites from the western coastline of the USA and analyzed for 15 analytes indicates that the data structure could be explained by four latent factors. These factors are conditionally named “anthropogenic”, “organic”, “natural”, and “hot spots”. They explain over 85% of the total variance of the data system, which is an acceptable value for the PCA model. The receptor models obtained after regression of the mass on the absolute principal components scores ensures reliable estimation of the contribution of each possible natural or anthropogenic source to the mass of each chemical component. It can be concluded that the region of interest reveals a different pattern of pollution compared with the eastern coastline treated statistically in a previous study.
Quantification of butanol and ethanol in aqueous phases by reflectometric interference spectroscopy – different approaches to multivariate calibration by F. Dieterle; D. Nopper; G. Gauglitz (pp. 723-730).
This paper presents several methods for analysis of data from reflectometric interference spectroscopic measurements (RIfS) of water samples. The set-up consists of three sensors with different polymer layers. Mixtures of butanol and ethanol in water were measured from 0 to 12,000 ppm each. The data space was characterized by principal component analysis (PCA). Calibration and prediction were achieved by multivariate methods, e.g. multiple linear regression (MLR), partial least squares (PLS) with additional predictors, and quadratic partial least squares (Q-PLS), and by use of artificial neural networks. Artificial neural networks gave the best results of all the calibration methods used. Calibration and prediction of the concentration of the two analytes by artificial neural nets were robust and the set-up could be reduced to only two sensors without deterioration of the prediction.
Temperature dependence of the optical properties of CuMoO4 by G. Steiner; R. Salzer; W. Reichelt (pp. 731-734).
CuMoO4 crystals reversibly change their color from green to brown upon heating, accompanied by a loss in transmittance. UV/VIS spectroscopic analysis revealed that these changes are due to particular electronic properties of the crystal instead of its chemical decomposition or structural change. Investigations were carried out in the temperature range 23–400 °C. The intensive green color of the crystal at room temperature is caused by a small transmission window between two absorption bands, the band gap of the crystal in the blue and the 3d9→4p absorption of the Cu2+ ions in the red. With increasing temperature the band gap shifts towards longer wavelengths, and the crystal changes both color and transmittance. Spectroscopic features of the crystal are discussed together with the temperature dependence of its electrical resistance. Resistance measurements were performed simultaneously to the optical measurements.
Electrolytic hydride generation atomic absorption spectrometry for the determination of antimony, arsenic, selenium, and tin – mechanistic aspects and figures of merit by E. Denkhaus; F. Beck; P. Bueschler; R. Gerhard; A. Golloch (pp. 735-743).
This article deals with the electrocatalytic and electrochemical mechanisms of hydride formation and their dependence on hydrogen overvoltage. A three-electrode-arrangement was used to determine the hydrogen overvoltage of different cathode materials (Pt, Au, Ag, glassy carbon, Cd, Pb, amalgamated Ag). The applicability of these cathode materials was tested for hydride formation using As(III), As(V), Sb(III), Sb(V), Se(IV), and Sn(IV). Glassy carbon is the most suitable cathode material for hydride generation with As(III), Sb(III), Se(IV), and Sn(IV). Hg–Ag is well suited for the production of stibine and arsine. As(III), As(V), Sb(III), and Sb(V) were all converted into their hydrides with efficiencies > 90%. A detection limit in the range of 0.11–0.13 μg L–1 for As and Sb (sample volume 200 μL) was obtained for cathode materials with a high hydrogen overvoltage. The precision of replicate measurements was better than 5% calculated as variation coefficient. The accuracy of the presented method was verified by analysis of certified reference materials and tissues of cancer patients. The recovery rates for As and Se were calculated to be 93–108%.
Rapid and sensitive determination of radiocesium (Cs-135, Cs-137) in the presence of excess barium by electrothermal vaporization-inductively coupled plasma-mass spectrometry (ETV-ICP-MS) with potassium thiocyanate as modifier by Ming Song; Th. U. Probst; Natalia G. Berryman (pp. 744-751).
An electrothermal vaporization-inductively coupled plasma-mass spectrometric (ETV-ICP-MS) method based on selective volatilization of cesium with KSCN as modifier has been developed for determination of radiocesium, i.e. 135Cs and 137Cs, in the presence of isobaric barium. A 10000 times excess of barium, which was volatilized at a temperature of 1100 °C, resulted only in a 1% signal increase in the signal of mass 135 amu. The recommended concentration of KSCN is 0.3 mM, and pretreatment and volatilization temperatures are 400 °C and 1100 °C, respectively. A ramp time of 1 s is recommeded for the volatilization step. The achieved limit of detection for 135Cs is 0.2 pg/mL (10 μBq/mL) and 4 fg (0.2 μBq) absolute for a sample volume of 20 μL. This means a limit of detection for 137Cs of 0.2 pg/mL (0.6 Bq/mL) and of 4 fg (0.01 Bq) absolute. Signal variations of 135Cs and 137Cs, respectively, in spiked samples with various matrices were investigated.
Characterization of municipal solid waste incineration (MSWI) bottom ash by scanning electron microscopy and quantitative energy dispersive X-ray microanalysis (SEM/EDX) by C. Speiser; T. Baumann; R. Niessner (pp. 752-759).
Scanning electron microscopy (SEM) with energy-dispersive X-ray microanalysis (EDX) is frequently used for morphological and qualitative chemical characterization of different materials. The applicability of this method for phase identification, is, however, often underestimated. The application of SEM/EDX for the characterization of different phases in fresh and altered municipal-waste incinerator bottom-ash samples with high lateral resolution is presented. Polished thin sections were prepared from the samples, but fresh fracture surfaces were also used. The EDX analyses were performed by using the correction procedures of a conventional standardless ZAF correction, a peak-to-background ZAF correction, and a correction method for light-element analysis. Because of their highly reactive properties the bottom-ash SEM samples require a special method of preparation. The method facilitates nondestructive preparation of the sensitive bottom-ash alteration phases (e.g. cement phases, hydroxides, salts) and their microstructures.
Direct solid sample analysis of geological samples with SS-GF-AAS and use of 3D calibration by G. Nimmerfall; Werner Schrön (pp. 760-767).
These results from direct solid-sample trace-element analysis, with 3D calibration, follow those of earlier publications. Cd, Cr, Cu, Ni, Pb, and Zn have been determined in complex inorganic powder samples, e.g. sediments, soils, rocks, and minerals. The measurements were performed with two SS-GF-AAS spectrometers which differ in the type of graphite furnace (transverse and longitudinal tube) and the background-correction technique (with and without the Zeeman system). The performance of the 3D calibration method was optimized by systematic investigation and the 2D- and 3D calibration techniques were compared. Another feature is the determination of confidence limits of the 3D calibration. This calibration error was calculated by the statistical bootstrap method. The results of calibration experiments and the precision and accuracy of the determination of trace elements in CRMs show that 3D calibration becomes increasingly essential as the refractory character of the analyte increases. Although there are large differences between the state of development and technical performance of the instruments used, 3D-calibration works with both SS-GF-AAS spectrometers.
Speciation of beryllium, nickel, and vanadium in soil samples from Csepel Island, Hungary by M. Óvári; M. Csukás; Gy. Záray (pp. 768-775).
Sequential leaching methods have been used for the speciation of Be, Ni, and V in five soil samples from Csepel Island on the Danube river located near an oil-fired power plant. The concentrations of the elements of the extracts were determined by inductively coupled plasma mass spectrometry. The total efficiency (the sum of the extracted metals divided by metal fraction which is soluble in aqua regia) of the five-step method was always higher. The difference for Ni was considerable, because of the high abundance of Ni found in the moderately reducible fraction, which is absent from the three-step method. The sum of the mobile species (exchangeable, carbonatic, and easily reducible) determined by both methods, were in reasonable agreement; this was not so for the individual fractions. There were greater differences between the non-mobile fractions (moderately reducible and oxidizable), because of the presence or absence of the moderately reducible fraction. For both methods there was good correlation between the oxidizable fraction and the organic matter content of the soils.
Solid-phase reactors in sequential injection analysis. Determination of manganese(II) in tap water and effluent streams using a solid-phase lead(IV) dioxide reactor in a sequential injection system by E. B. Naidoo; J. F. van Staden (pp. 776-780).
The determination of manganese(II) in tap water and effluent streams, using a solid-phase reactor incorporated into a sequential injection system was investigated. Mn2+-ions in samples injected into a carrier stream, were oxidised by solid lead(IV) dioxide suspended on silica gel beads to form MnO4 –-ions which were detected spectrophotometrically at 526 nm. The linear range of the system is from 1 to 7 mg L–1 with a detection limit of 0.62 mg L–1. The proposed system is suitable for the determination of manganese(II) in tap and effluent streams at a rate of approximately 50 samples per hour with a relative standard deviation of better than 3%. Statistical comparison between the proposed sequential injection system and a standard ICP method revealed that there is no significant difference between the two methods at the 95% confidence level for effluent streams and at 99.9% for tap water.
Capillary zone electrophoresis of proteins with poly(2-hydroxyethyl methacrylate)-coated capillaries: fundamentals and applications by Felix C. Leinweber; Johann Stein; M. Otto (pp. 781-788).
Fused silica capillaries have been modified by atom-transfer radical polymerization (ATRP) to generate covalently bonded polymer films of 2-hydroxyethyl methacrylate. Because the kinetics of ATRP have mainly been investigated in bulk solutions, a GC experiment was set up to examine monomer conversion inside narrow-bore capillaries. It was shown that after 1 to 4 h the reaction was nearly complete. The coating process was further optimized by monitoring EOF, because low EOF indicates high surface coverage. To deal with the very low EOF values, a new approach was used to dramatically reduce the measurement time by overlaying hydrodynamic flow on the electroosmotic flow. The corresponding equations are derived separately in detail. Capillaries were then coated under optimum conditions with linear or cross-linked polymer films. The EOF was reduced over a wide range of pH values. A long-term reproducibility test with both types of functionalization showed that the efficiency of the linear polymer coating decreased significantly over time. With cross-linked films, however, the efficiency even increased. Relative standard deviations for protein migration times were also much lower in cross-linked coated capillaries. Highly efficient separations could be performed for basic and acidic proteins in acidic media, and for the latter even in basic media.
Raman spectroscopic surface characterization of cellulose derivatives by N. Meyer; K. Nestler; L. Engisch; G. Marx; M. Füting; A. Kiesow; A. Heilmann; S. Wartewig; P. Kleinebudde (pp. 789-791).
First results of experiments on the surface characterization of cellulose derivatives are presented. Different water contents of the surface of microcrystalline cellulose have been investigated by means of Raman spectroscopy, SERS, and environmental scanning electron microscopy (ESEM).